I have used the small ones for 1-1.5 gallon test batches and they work fine for a primary. They clean up well with an overnight oxyclean soak and some shaking. I drilled a hole in the lid and fit it with an airlock. I also got smaller ID tubing to rig up to a bottle filler so I can go straight from the spigot to bottles with carb tabs. I primary only on my test batches, and most everything else I brew. I never use plastic for secondary, only glass, but that is a debate for another day.

I was even thinking for those who keg and want to take some brew to a friends party or cookout. Load her up and be able to keep the beer cold and serve with the spigot. Won't have to mess with growlers which aren't totally fridge friendly.